| This gives practice in the use of Ohm's Law and Kirchoff's Laws. |
| The coil in a typical moving
coil meter has a resistance of 100 ohms.
The current to deflect the pointer fully across the scale is called full scale deflection (f.s.d.) and is typically 1 mA. From Ohm's Law, the voltage for f.s.d. is I x R = 0.001 x 100 = 0.1 volts. If we want the meter to read 1 volt at f.s.d. then Mr Kirchoff says that we need to drop 0.9 volts across a series resistor (MULTIPLIER). Since there is 0.001 Amp through the multiplier, the value is , from Ohm's Law, 0.9V/0.001A = 900 ohms, |
| Using several multipliers and a selector switch, we can make a multi-range voltmeter. |
Copyright Graham Knott 1999